The current version of Calibre - 0.8.51 - has a significant regression which prevents it from processing .Lit files, which is one of the most common ebook format. Since Calibre's main function is to manage ebooks and convert them between various formats for use on ebook readers, this is a pretty significant bug. The regression was fixed in verion 0.8.52 and onward.

Currently, version 0.8.60 is pending migration to Testing, but cannot migrate due to a block request by freeze. The instructions say to contact you to request the block be removed.

Leaving Calibre at the current version means keeping a crippled version. Imagine a car that can go forward, backward and turn left. Right turns, however, well.... not so much.